The present invention relates in general to a limited slip differential for a vehicle and, in particular, to an hydraulically actuated variable lock differential.
Early differential mechanisms consisted of a set of planetary gears coupled between two half-shafts of a drive axle. Such a drive axle has the advantages over a solid axle that the wheels of the vehicle can travel at different speeds and equal driving force can be applied to the driving wheels. However, under certain conditions, this conventional differential has a serious deficiency. For example, if one drive wheel is on a slippery surface, such as ice or mud, that wheel will slip and spin because its tire can not grip the road. Consequently, the slipping wheel can apply very little driving torque to move the car. The opposite drive wheel, which well may be on a surface that gives good adhesion, can apply no more driving torque than the spinning wheel because the differential delivers only an equal amount of torque to both wheels. Thus, the total driving force can never be more than twice the amount applied by the wheel with the poorest road adhesion.
Traction is also adversely effected, especially during hard driving, by other conditions that unbalance the weight on the driving wheels. When driving at high speed around a curve, the weight is transferred from the inside wheel to the outside wheel. Hard acceleration coming out of a turn can then cause the inside wheel to spin because it has less weight on it and therefore less road adhesion. Similarly, during any hard acceleration there is propeller shaft reaction torque on the rear axle assembly. When one wheel is partially unloaded and looses part of its traction capability, the loss is not offset by gain on the opposite side because the total can only be twice that of the wheel with the lesser capability.
The limited slip differential was designed to improve the traction of a vehicle under adverse traction conditions by allowing the differential to transmit torque to the axle shafts in unequal amounts without interfering with the differential action on turns. The most common limited slip differential is the friction type which has clutch assemblies mounted between the two side gears and the differential case. In a conventional differential, the side gears and the axle shafts to which they are splined always turn freely in the case. The added clutches provide a means of transferring torque from the faster spinning (usually slipping) wheel to the slower spinning (usually better adhesion) wheel.
Typically, there are two clutch packs, each of which is comprised of disks that are splined to the side gear, and plates that are tanged to fit into the differential case. Thus, the disks rotate with the side gear and the plates rotate with the case. The clutches are applied or actuated by two forces. One force is applied by springs compressed between the two side gears which push the side gears apart, towards the case, and thus keep the plates and disks in contact with each other. This force is relatively constant and preloads the clutches. The other force results from the tendency of the pinions and side gears to push themselves apart as they turn. This force is applied through the side gears and increases the pressure on the plates and disks. This force becomes greater as the driving torque transmitted from the pinions to the side gears increases and is therefore a variable force.
The typical limited slip differential has a design limit on the amount of torque transfer from the faster to the slower wheel, so that the torque on the wheel with good traction is about two and one half times that of the wheel with poor traction. From the above description, several shortcomings of the common limited slip differential are apparent:
(1) During turning maneuvers, torque is transferred to the inside wheel at a rate generally proportional to the driving torque. This results in a tendency to understeer.
(2) Under conditions where one driving wheel is on a very slippery surface while the other has good traction, the amount of torque that can be transferred is very limited, essentially determined by the preload spring force on the clutch packs.
It is the intent of this invention to overcome these shortcomings by providing an externally controllable limited slip differential whose clutch actuating force is not dependent on preload springs or side gear separating forces caused by drive line torque, but rather is provided by hydraulic pressure. This pressure may be regulated as necessary to adjust the differential from zero to full locking as driving needs dictate.